Previous field studies suggested that when, seasonally, the external environment begins to degenerate, infective hookworm larvae are physiologically modified so that their subsequent development in the host includes a dormant state lasting several months. Thereafter, a strain of Ancylostoma duodenale was shown to remain dormant in man for 8-9 months. Investigations reported herein demonstrate that certain external environmental factors can stimulate hookworm larvae to arrest in dogs, and that these larvae do resume development. These are the major attributes of a suitable model system for the study of arrest. It is the broad objective of this proposal to identify factors involved in: 1) the manifestation of developmental arrest among hookworms; 2) the retention of arrested larvae in the host; 3) the release of the larvae from dormancy. Investigations of these factors in relation to developmental arrest of Ancylostoma caninum in pups will be continued. Studies on the laboratory maintenance of Ancylostoma duodenale in dogs will be initiated. As it is inferred that this species may arrest in dogs, as it does in man, preliminary investigations on the site of arrest will be undertaken, and dogs will be observed for intervals of time permitting resumption of development to be observed. In relation to the manifestation of arrested development, proposed investigation will consider the immunological responsiveness of the host. The latter will also be examined as regards retention of arrested larvae and their subsequent release from dormancy. Hormonal changes and intercurrent infections may also be causally associated with resumption of larval development, and their roles in the process of reactivation will be investigated. BIBLIOGRAPHIC REFERENCES: Schad, G. A. 1977. The role of arrested development in the natural regulation of nematode populations. In Regulation of Parasite Populations, G. W. Esch, Editor. Academic Press, New York pp. 111-167. Nawalinski, T., Schad, G. A. and Chowdhury, A. B. 1977. Dynamics of hookworm populations in children in rural West Bengal. I. General Parasitological Observations. Amer. J. Trop. Med., in press.